All the experts on Getting Your Work Published say that you can't have mistakes. One grammar error, one misspelling, one comma out of place, and that's it. The editor tosses it in the trash without giving it a chance.
I've never tossed a Dark Moon Digest submission for grammatical errors. The thing about mistakes is that everybody makes them and they're really easy to correct. I'm not going to cheat a publication out of a really good story by dismissing it because of something so superficial. This from the woman who carries a Sharpie around with her so that she can correct grammar on the go.
On the other hand, bad grammar in ad copy, on a website, or in any other professional communication prejudices me against a business like nothing else. If you're so careless with your copy, what else are you careless about? If you don't put any effort into making sure your copy free from glaring errors, I don't think it's unreasonable to be concerned that you might not put much effort into customer service, product reliability, or safety.